Modern electronic prepress operations utilize laser scanning systems to write or record images for subsequent reproduction or to scan a prerecorded image at a predefined resolution rate. Such scanning systems may write or record images or scan prerecorded images on various prepress media including, photo or thermal sensitive paper or polymer films, photo or thermal sensitive coatings or erasable imaging materials mounted onto an image recording surface or photo or thermal sensitive paper, polymer film or aluminum base printing plate materials, all used in electronic image reproduction. Such media are mounted onto a recording surface which may be planar but which is more typically curved and scanned with a recording or scanning beam or beams. The primary components of such a system include a recording surface, usually a drum cylinder and a scan mechanism disposed and movable within the drum cylinder or drum movable relative to scan mechanism. The system also includes a processor, with an associated storage device, for controlling the scanning mechanism and for scanning a prerecorded image, a photodetector and detector processor. The processor and associated storage device may be housed within the system itself or separate from the system with appropriate interconnection to the system.
The processor, in accordance with stored programming instructions, controls the scanning mechanism to write or read images on the plate or other medium mounted to the inner drum cylinder wall by scanning one or more optical beams over the inside circumference of the drum cylinder while the drum cylinder itself remains fixed.
The scanning and hence the recording are performed over only a portion of the cylinder inner circumference, typically between 120 and 320 of the circumference of the drum cylinder. The optical beam(s) are typically emitted so as to be parallel with a central axis of the cylinder and are deflected, by for example, a spinning mirror, Hologon or Penta-prism deflector so as to form a single scan line or multiple scan lines which simultaneously impinge upon the recording surface. The deflector is spun or rotated by a motor about an axis of rotation substantially coincident with the central axis of the drum cylinder. To increase the recording speed, the speed of rotation of the beam deflecting device can be increased. To even further increase the recording speed, multiple beam scanning has been previously proposed.
One such proposed multiple beam scanner has utilized a spinning dove prism with a single light source, as discussed, for example, in U.S. Pat. No. 5,214,528. Using a dove prism beneficially allows the use of a multiple beam source, e.g. a laser diode array, while eliminating the need for multiple beam correction elements and associated hardware. Additionally, for reasons which need not be discussed here, the scan speed of multiple beam systems using a dove prism can exceed that of other types of proposed multi-beam systems.
In a typical multibeam scanning system, a dove prism is disposed in the optical path between the beam source and the deflector. The prism is caused to rotate about an axis coincident with the rotational axis of the deflector (or an optical axis which becomes coincident therewith) at half the rotational speed of the deflector. Since the rotation of the dove prism produces a 2.times. axial rotation of all light beams passing through the prism, the multiple beams leaving the prism will rotate in lock step with the rotation of the deflector. Accordingly, by passing the multiple light beams through a spinning dove prism, crossing of the multiple scan lines formed by the spin mirror is avoided. For a more detailed description of the operation of a dove prism with respect to a multibeam scanning system, reference may be made to U.S. Pat. No. 5,214,528.
In scanning systems of the foregoing description, beam positioning errors adversely affecting image quality will result if the rotation axes of the dove prism (or similar beam rotation element) and of the deflector are not perfectly aligned. Misalignment of the dove prism and deflector rotational axes may be caused by, for example, a wobble or other dynamic anomaly associated with the dove prism shaft and/or deflector shaft, or an error in mounting the dove prism and deflector relative to one another. Rather than forming straight scan lines on the scanning surface, a scanning system having a misalignment will produce scan lines that are bowed or have an otherwise curved aspect. Curving or bowing of the scan lines, in the aggregate, may substantially compromise the quality of the scanned image.
Furthermore, because the prism rotates at half the rotational speed of the deflector, beam misalignment arising from a wobble or other dynamic anomaly, can cause a twinning effect between groups of multiple beams. If the twinning effect is excessive, the system will be restricted to scanning only every other rotation of the deflector to avoid twinning and thereby obtain scans of acceptable quality. Thus, a two beam system would have an effective scanning rate equal to that of a single beam system, a four beam system will have an effective scan rate only twice as fast as a single beam system, and so on.
It is possible to reduce the aforementioned beam positioning errors through various techniques, such as utilizing tight mechanical tolerances with respect to mounting of the dove prism and deflector, or by increasing the ratio of the beam diameter at the dove prism to the diameter at the deflector. However, these techniques are generally insufficient to completely eliminate beam mispositioning with respect to the scanning surface, and imaging artifacts will continue to be present even if such techniques are employed.
U.S. Pat. No. 5,097,351 discloses a multibeam system which employs a controlled movable reflector in place of a dove prism, and requires that each of two beams follow a separate optical path, each path having separate focussing and collimating optics and acousto-optic modulators. The controlled reflector is positioned in only one of the optical paths and is driven to rotate the beam in synchrony with the rotation of the spin deflector. Beam positioning errors, detected by a quadrature photodetector array, are corrected by driving the reflector to adjust angular alignment during scanning operations. However, the complexity of the foregoing system makes practical implementation thereof a difficult proposition.